The invention concerns simultaneous acquisition of transmitted counts and emitted counts for a gamma camera. It concerns, among others, scintillation cameras of the ANGER type for which document U.S. Pat. No. 3,011,057 describes its operating principles and its means of achievement.
In nuclear imaging systems, an image can be obtained in two different ways. A first method consists in placing a radioactive source opposite a detector and in placing the body or the object to be examined between the source and the detector. The result obtained using this first method is commonly called the "transmission image" by a person skilled in the art. This name comes from the fact that the radiations are transmitted through the body or the object. A second method consists in placing a detector opposite the object or the body to be examined. For a human body, one will have previously injected a radioactive isotope into the patient before the examination. The image obtained with this second method is commonly called the "emission image" as, with this method, the image is formed from the radiations emitted by the body under examination.
In nuclear medicine, the two types of images fulfil two different functions. The transmission image gives an image of the inside of the body which depends on the transparency of the various organs in relation to the radiation. The emission images however are used to display certain organs that it would be impossible to observe by transmission or to determine the operation of an organ in relation to the distribution of the radioactive isotope.
In an emission image, the distribution of the isotope depends on the fixation of the said isotope in the various organs, and the fixation depends on the type of isotope used and the various characteristics of each organ. Therefore, radiations are potentially emitted from all points of the human body. A part of these radiations must therefore pass through a part of the body. The emission image is altered by the scatter of gamma-ray photons passing through certain parts of the body under examination, causing a blurred image.
A solution to this problem consists in correcting the emission image by means of a transmission image. However, to be used to correct the emission image, the transmission image must be obtained under the same conditions as the emission image.
Gamma cameras can validate a gamma-ray photon impact (commonly called count) which occurs on their detectors as a function of the energy level of the said count received. A method consists in taking images simultaneously by separating the counts received as a function of the energy levels. A first radioactive isotope producing counts at a first energy level (for example 140 keV technetium) is injected into the patient, and a homogeneous source containing a second radioactive isotope producing counts at a second energy level (for example 100 keV gadolinium) is placed behind the patient. When the gamma camera detector receives a count, it is possible to detect whether it is a transmission or an emission according to the energy received.
Such a method has imperfections. The counts produced by the isotopes have a certain energy scatter requiring a certain width for the energy window to determine to which energy category the count belongs; in general, this window is designed with tolerances of 10 to 20% on the nominal energy which can vary as a function of the isotope in order to avoid an overlong exposure time for the patient. In addition, certain counts are converted into Compton diffusion with lower energy, the counts counted in the lower energy level recording are the Compton photons corresponding to the higher energy level counts. Concerning the accuracy of the transmission image, it is preferable to use isotopes as close as possible for the emitted count energy levels as the transmission phenomenon depends, among other things, on the energy of the photons.
To sum up, taking two images simultaneously tends to degrade the quality of both of the images. Solutions have been found to attempt to reduce the undesirable effects. It is possible to take images successively but this increases the duration of the examination and the risk of the patient moving. It is also possible to use a collimated source with a surface area lower than the size of the detector that is moved behind the patient in order to obtain a less degraded emission image.